This invention relates to lattice panel structures, such as bridges of the xe2x80x9cBaileyxe2x80x9d type. The invention is particularly concerned with a modular lattice panel system.
In a typical xe2x80x9cBaileyxe2x80x9d bridge construction there is provided a main girder at each side of the bridge, transoms extending between the girders and a deck supported on the transoms. The girders are formed from prefabricated lattice panels of steel which are joined together longitudinally. Two or more sets of the lattice panels may be secured together in the vertical direction so as to provide the required depth of girder, and to this end the panels are generally of a rectangular configuration. A common type of lattice panel consists of upper and lower longitudinally extending chords which are joined together by a lattice of web members. These will generally include vertically extending web members and angled web members which may for example extend at an angle of about 45xc2x0 to the chords. Various panel configurations are disclosed in GB-A-2 251 018, for example.
To join the panels together in end to end relationship to provide the required length of girder, pin and aperture joints are usually employed to ensure speed and simplicity of assembly on site. One end of each chord will be formed with a female portion and one with a male portion. These are mated when the panels are to be joined together and then a transverse pin is inserted through apertures in the portions.
Bridges of the above type are often used as temporary structures, for example to replace bridges destroyed by floods, earthquakes or acts of war. They are supplied as prefabricated components which are assembled on site. In one common method of construction, the bridge girders are assembled on one side of the gap to be bridged, such as a river or ravine, and pushed out and over to the other side.
In a conventional system, the lattice panels are provided as completely prefabricated units with the web members welded to the chords. Typically, a standard length prefabricated panel will be available, and the designer of a bridge will use the appropriate number of these, to be joined end to end, for the bridge.
A significant advantage of using prefabricated panels is that bridges can be constructed quickly with the minimum of on-site fabrication however, there are design constraints due to the limited number of panel configurations available. It is also necessary to restrict the length of the prefabricated panels used as the basic units, so that there is sufficient flexibility to achieve a desired length by joining a number together. however, the joins between adjacent panels can be expensive, particularly if they are of the pin joint variety requiring forgings. The more panel units are required to span a given length, the more joins are required a further problem is that prefabricated panels are bulky to transport.
An object of the present invention is to provide a modular system for constructing lattice panels which provides greater flexibility but which does not increase to an undesirable level the time spent on site to construct a bridge.
Viewed from one aspect the present invention provides a modular system for the construction of a lattice panel for a structure, comprising first and second elongate chord members and a plurality of web members for attachment to the chord members so as to hold them apart transversely, wherein each web member comprises three legs forming a triangle, a first leg being adapted to extend perpendicularly between the chord members, and second and third legs being joined to each other and to adjacent the ends of the first leg, and wherein there are provided the following means for interlocking each web member to the chord members and to like web members:
first interlocking means adjacent one end of the first leg to interlock with corresponding second interlocking means provided at intervals along the first chord member;
third interlocking means adjacent the other-end of the first leg to interlock with corresponding fourth interlocking means provided at intervals along the second chord member;
fifth interlocking means adjacent the join between the second and third legs; and
sixth interlocking means intermediate the ends of the first leg;
the arrangement being such that the fifth interlocking means of one web member is adapted to interlock with the sixth interlocking means of an adjacent like web member.
Thus, in use a lattice panel can be constructed with upper and lower chord members and a lattice of web members providing vertical legs extending between and connected to the chord members, and inclined legs which are joined to the vertical legs of adjacent web members. Such an arrangement provides a strong lattice panel.
It is a simple matter to provide lattice panels of desired lengths, by selecting chord members of appropriate lengths and a corresponding number of web members. To cover a certain distance it may be possible to use a single panel constructed from the modular components with long chord members, rather than two or more standard lattice panels joined end to end. From a manufacturing point of view, it is preferable to fabricate and store the modular components capable of forming lattice panels of various lengths, rather than complete lattice panels of various lengths. Furthermore, the conventional joints between lattice panels, typically involving forged components, are expensive. The ability to construct longer panels, simply and from prefabricated components, reduces the total number of panels required for a particular job and thus the number of expensive joints The end user will also have fewer inter-panel joints to assemble if longer panels can be used.
The fabrication of the lattice panels may be carried out at a manufacturing site once an order is received, for shipment to a place of use. Alternatively, the modules may be shipped to the end user for assembly into panels on-site. This may be preferable from a shipping point of view. It may also be possible for the chord members to be manufactured locally if their design is simple enough, this being discussed below, so that only the web members have to be shipped.
It is known for the upper chords of lattice panels to be joined by simple compression joints, for example using abutting flanges and threaded fasteners. These chords can be made on site relatively easily. The lower chord joints are in tension and conventionally they have been in the form of pin joints which provide the required tensile strength whilst being relatively quick and simple to assemble. The pin joints are expensive, forged items, and it is less feasible to manufacture the lower chords on site. In accordance with the present invention, however, it is practical to use longer chords and fewer chord joints. For example, a panel in accordance with the invention may be between three and four times the length of a conventional panel. It is thus feasible to use alternative chord joints for the lower chords, which are cheaper and easier to manufacture even though it may take longer to join two chords. Thus, in one proposed arrangement the lower chord joints are provided by splice plates and several threaded fasteners. An advantage of such a simple joint is that the chords only need to be provided with apertures for the fasteners and it is a more practical proposition for the chords to be made on site.
There may be a number of different chords that can be used For example, a stronger chord could be provided if required. This could be only at suitable positions, such as at the centre of a bridge span. This avoids the need to take a standard lattice panel and add a reinforcing chord to it. It is also possible to introduce camber by using chords of different lengths at the top and bottom of a panel. A longer upper chord will introduce positive camber, resisting the tendency of a bridge to sag in the middle.
The interlocking means should be such that the modules can be assembled in the required orientation and also provide for the transfer of forces. In a preferred embodiment, where two interlocking means interlock, there is provided a male spigot on one member and a matching female recess, for example in the form of an aperture, on the other member. For any interlock there may be one, two, three, four or more such spigots and matching recesses. There may also be fasteners such as threaded bolts secured by nuts to clamp the components together, although the primary purpose of such threaded fasteners will generally not be to transfer forces. In one preferred form, the first and third interlocking means, at opposite ends of the first leg of the web member, comprise spigots. This means that the chords only need to have apertures to constitute the corresponding second and fourth interlocking means. This again helps in simplifying the design of the chords, reducing manufacturing costs and also making it feasible for them to be manufactured on site. However, if desired, reinforcing components could be attached to the chords to receive the spigots.
The fifth and sixth interlocking means, which are provided to join the web members together, preferably also comprise spigots and recesses. For ease of manufacture, at least one of the interlocking means may be provided on a cast member.
In general, the philosophy behind the preferred implementation of the invention is to keep the chords and their joints as simple as possible, and to concentrate the more complex and/or expensive structures on the web members. The design of the modular web member may lend itself to robotic construction, something which has not been considered feasible with the construction of entire lattice panels from individual components.
A web member may be in the form of an isosceles triangle, with the second and third legs of equal length. In such an arrangement, the sixth interlocking means will be arranged on the mid point of the first leg and the fifth interlocking means, where the second and third legs meet, will be aligned with this. In one preferred arrangement, the angle that each of the second and third legs makes with the first legs is about 45xc2x0, so that the width of a web member is about one half of its height, i.e the extent of the first leg which extends perpendicularly between the chord members. By using a web member with a longer first leg, it is possible to construct a deeper lattice panel. Preferably, when this is done the angles between the first leg and the second and third legs are preferably reduced to retain the same width for the web member so that it will remain compatible with the same chords, decks and other components used with other web members. In general, the angle between the first leg and the second/third legs is preferably in the range of 35xc2x0 to 45xc2x0. In preferred embodiments, this range can provided panels in the range of about 15 feet (about 4.5 m) to about 23 feet (about 7 m) high Conventional Bailey bridge panels are frequently stacked on each other to increase height, and apart from anything else this doubles the number of chord joints that have to be made.
It will be appreciated that in a practical arrangement the triangle may not be perfect and that the legs might not consist solely of members which meet immediately adjacent their ends, for example. Thus, in one preferred arrangement the second and third legs may be joined together by a junction unit which receives the ends of both legs and is provided with the fifth interlocking means. Similarly, the first leg may comprise a member which is joined at each end to a junction unit. These are respectively attached to the second and third legs, and have the first and third interlocking means for connection to the chord members.
A typical chord member for use in accordance with the present invention be of H section. Such a section will effectively define a pair of channels. One end of a first leg of a web member (in practice, a junction unit) will be received within one channel section of an upper chord, and the other end of the first leg will be received within one channel section of a lower chord.
The web member first legs may each comprise a pair of spaced, parallel elements. These will help to resist outwards deflection of the chords, and in particular the upper chord when a panel is used in a bridge. The spaced elements are preferably tube members, as indeed are the legs of the web members generally. The use of such web members, resistant to deflection, means that there will normally need to be only one line of panels along a side of a bridge, with only a single upper chord and a single lower chord. Frequently in traditional Bailey bridge structures it is necessary to have twin lines of panels. This therefore doubles the number of chord joints and this is another reason why in preferred embodiments of the present invention it is feasible to use chord joints which, individually, take longer to assemble. The preferred web members, being more resistant to deflection, may also make it unnecessary to use additional lateral struts which are frequently used in conventional Bailey bridge structures.
The chord members may be provided with any suitable means for interconnecting them to the chord members of adjacent panels. This include male and female pin joint portions, for receiving either vertical or horizontal pins; apertured plates for receiving bolts or other suitable fasteners as disclosed in GB-A-2 251 018 for example; or any other suitable means. However, as noted earlier, a preferred joint for the upper chords is a compression joint using flanges and fasteners, and a preferred joint for the lower chords is a tension joint using splice plates and fasteners. This also has the advantage that such a joint may more resistant to fatigue, as it does not require the use of welding to attach forged pin joint components to the chords.
When a lattice panel is constructed using the chord members and web members as described above, at one end there may be the second and third legs of a web member projecting beyond the ends of the chords; and at the other end there will then be a first leg of another web member positioned inwardly of the ends of the chords. Joining two lattice panels together will involve joining the upper and lower chords together, and also joining the projecting web member to the web member of the adjacent panel.
For use in a bridge or similar structure, preferably the first leg of a web member is provided with means for attachment to a transom which will support a deck. The connection between the leg and the transom may be by means of a spigot and recess, for example a trapezoidal cross section recess on the first leg and a matching spigot on the transom. In practice, it may only be necessary to attach a transom to alternate web members. The web members which are not to be attached to transoms may not be provided with the necessary attachment means, and they may even have lighter first legs as they will be required to withstand less stress than the first legs of the other web members. Where s transom is attached, the upright first legs form the uprights of a stress transmitting xe2x80x9cUxe2x80x9d. At the ends of a structure, there could be stronger web members with sturdier upright legs and if desired also sturdier diagonal legs, to account for increased shear forces. These end web members could be provided with means for attachment to transoms. Other web members could be provided for various purposes as required. For example, a special web member adapter could be provided so that a bridge can be launched using a conventional launcher nose used with current Bailey type bridges.
The invention may be viewed from various different aspects, dealing with the system in broad terms, a web member for use in the system, novel lattice panels constructed using the system, a bridge or other lattice panel structure such as a tower constructed using the system, a method of constructing such a structure, and so forth.
For example, viewed from one aspect the present invention provides a prefabricated web member for use in a system as described above, comprising three legs forming a triangle, wherein:
a first leg comprises an elongate member having at each end respective first and second mounting plates perpendicular to the axis of the first leg, each mounting plate being provided with first interlocking means for connection to a chord member and with at least one aperture to receive a fastener to secure the mounting plate to the chord member;
the second and third legs extend at an acute angle from adjacent the ends of the first leg to a junction where there is provided a third mounting plate whose plane is parallel to the longitudinal direction of the first leg, the third mounting plate being provided with second interlocking means for connection to another web member and with at least one aperture to receive a fastener to secure the mounting plate to the other member; and
the first member is provided with a fourth mounting plate intermediate its ends whose plane is parallel to the longitudinal direction of the first leg, the plate being provided with third interlocking means for connection to the second interlocking means of another web member, and also being provided with at least one aperture to receive a fastener to secure the plate to the other web member.